Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing, or support layer; one or more unexposed photocurable layers; optionally a protective layer or slip film; and often a protective cover sheet.
The support sheet or backing layer lends support to the plate. Preferred materials for the backing layer include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. Generally the most widely used support layer is a flexible film of polyethylene terephthalate. The support sheet can optionally comprise an adhesive layer for more secure attachment to the photocurable layer(s). Optionally, an antihalation layer may also be provided between the support layer and the one or more photocurable layers. The antihalation layer is used to minimize halation caused by the scattering of UV light within the non-image areas of the photocurable resin layer. For most flexographic applications the support layer is clear, particularly when the establishment a photopolymer floor in the printing plate is desired.
The photocurable layer(s) include photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional relief pattern of cured material. Preferred photocurable materials include an elastomeric compound, an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. Photocurable materials are disclosed, for example, in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. More than one photocurable layer may be used.
The photocurable materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. The type of radiation used is dependent on the type of photoinitiator in the photopolymerizable layer. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety in the materials of the photocurable layer. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions. Any conventional sources of actinic radiation can be used for this exposure step, including, for example, carbon arcs, mercury-vapor arcs, fluorescent lamps, electron flash units, electron beam units and photographic flood lamps.
The slip film is a thin layer, which protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light. In this process, the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. Instead of a slip film, a matte layer may also be used to improve the ease of plate handling. The matte layer typically comprises fine particles (silica or similar) suspended in an aqueous binder solution. The matte layer is coated onto the photopolymer layer and then allowed to air dry. A negative is then placed on the matte layer for subsequent UV-flood exposure of the photocurable layer.
In a “digital” or “direct to plate” plate making process, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer, which is generally a slip film that has been modified to include a radiation opaque material. Portions of the laser ablatable layer are ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Examples of laser ablatable layers are disclosed for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photocurable material and reveal the crosslinked relief image in the cured photosensitive printing element. Typical methods of development include washing the printing element with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter. The resulting surface has a relief pattern that reproduces the image to be printed and which typically includes both solid areas and patterned areas comprising a plurality of relief printing dots. After the relief image is developed, the printing element may be mounted on a press and printing commenced.
A “back exposure” step may also be performed prior to imaging the photosensitive printing element (or immediately after imaging the photosensitive printing element). “Back exposure” refers to a blanket exposure to actinic radiation of the photopolymerizable layer on the side opposite that which does, or ultimately will, bear the relief. This step is typically accomplished through a transparent support layer and is used to create a shallow layer of photocured material, i.e., the “floor,” on the support side of the photocurable layer. The purpose of the floor is generally to sensitize the photocurable layer and to establish the depth of relief.
The shape of the dots and the depth of the relief, among other factors, affect the quality of the printed image. It is very difficult to print small graphic elements such as fine dots, lines and even text using flexographic printing plates while maintaining open reverse text and shadows. In the lightest areas of the image (commonly referred to as highlights) the density of the image is represented by the total area of dots in a halftone screen representation of a continuous tone image. For Amplitude Modulated (AM) screening, this involves shrinking a plurality of halftone dots located on a fixed periodic grid to a very small size, the density of the highlight being represented by the area of the dots. For Frequency Modulated (FM) screening, the size of the halftone dots is generally maintained at some fixed value, and the number of randomly or pseudo-randomly placed dots represent the density of the image. In both instances, it is necessary to print very small dot sizes to adequately represent the highlight areas.
Maintaining small dots on flexographic plates can be very difficult due to the nature of the platemaking process. In digital platemaking processes that use a UV-opaque mask layer, the combination of the mask and UV exposure produces relief dots that have a generally conical shape. The smallest of these dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (the dot is not “held” on plate and/or on press). Alternatively, if the dot survives processing they are susceptible to damage on press. For example small dots often fold over and/or partially break off during printing causing either excess ink or no ink to be transferred.
Furthermore, photocurable resin compositions typically cure through radical polymerization, upon exposure to actinic radiation. However, the curing reaction can be inhibited by molecular oxygen, which is typically dissolved in the resin compositions, because the oxygen functions as a radical scavenger. It is therefore desirable for the dissolved oxygen to be removed from the resin composition, and/or to stop atmospheric oxygen from dissolving in the resin, before image-wise exposure so that the photocurable resin composition can be more rapidly and uniformly cured.
The removal of dissolved oxygen may be accomplished in various ways. For example, the photosensitive resin plate may be placed in an atmosphere of inert gas, such as carbon dioxide gas or nitrogen gas, before exposure in order to displace the dissolved oxygen. Another approach involves subjecting the plates to a preliminary exposure (i.e., “bump exposure”) of actinic radiation. During bump exposure, a low intensity “pre-exposure” dose of actinic radiation is used to sensitize the resin before the plate is subjected to the higher intensity main exposure dose of actinic radiation. The bump exposure is applied to the entire plate area and is a short, low dose exposure of the plate that reduces the concentration of oxygen, which inhibits photopolymerization of the plate (or other printing element) and aids in preserving fine features (i.e., highlight dots, fine lines, isolated dots, etc.) on the finished plate. Other efforts have involved special plate formulations alone or in combination with the bump exposure.
U.S. Pat. Pub. No. 2014/0141378 to Recchia, the subject matter of which is herein incorporated by reference in its entirety, describes a method of imaging a photocurable printing blank in a digital platemaking process that includes the steps of laminating an oxygen barrier membrane to a top of a laser ablated mask layer and exposing the printing blank to actinic radiation through the oxygen barrier membrane and mask layer to selectively crosslink and cure portions of the at least one photocurable layer. The oxygen barrier membrane is removed prior to the development step. The presence of the oxygen barrier membrane produces printing dots having desired characteristics. The method can also be used with an analog platemaking process that uses a negative instead of an ablatable mask layer, or, in the alternative, the negative itself can be used as the oxygen barrier layer.
U.S. Pat. Pub. No. 2014/005/7207 to Baldwin, the subject matter of which is herein incorporated by reference in its entirety, describes the use of one or more UV LED assemblies in selectively crosslinking and curing sheet photopolymers can produce a relief image comprising flexo printing dots having desirable geometric characteristics.
As described in U.S. Pat. No. 8,158,331 to Recchia and U.S. Pat. Pub. No. 2011/0079158 to Recchia et al., the subject matter of each of which is herein incorporated by reference in its entirety, it has been found that a particular set of geometric characteristics define a flexo dot shape that yields superior printing performance, including but not limited to (1) planarity of the dot surface; (2) shoulder angle of the dot; (3) depth of relief between the dots; and (4) sharpness of the edge at the point where the dot top transitions to the dot shoulder.
Flexo plates imaged by typical digital imaging processes tend to create dots with rounded tops. A rounded dot surface is not ideal from a printing perspective because the size of the contact patch between the print surface and the dot varies exponentially with impression force. In contrast, a planar dot surface should have the same contact patch size within a reasonable range of impression and is thus preferred, especially for dots in the highlight range (0-10% tone).
A second parameter is the angle of the dot shoulder. The shoulder angle can vary depending on the size of the dots as well. There are two competing geometric constraints on shoulder angle—dot stability and impression sensitivity. A large shoulder angle minimizes impression sensitivity and gives the widest operating window on press, but at the expense of dot stability and durability. In contrast, a lower shoulder angle improves dot stability but makes the dot more sensitive to impression on press
A third parameter is plate relief, which is expressed as the distance between the floor of the plate and the top of a solid relief. The dot relief is to a certain extent linked to the dot's shoulder angle.
A fourth characteristic is the presence of a well-defined boundary between the planar dot top and the shoulder. Dots made using standard digital flexo photopolymer imaging processes tend to exhibit rounded dot edges. It is generally preferred that the dot edges be sharp and defined. These well-defined dot edges better separate the “printing” portion from the “support” portion of the dot, allowing for a more consistent contact area between the dot and the substrate during printing. Edge sharpness can be defined as the ratio of re, the radius of curvature (at the intersection of the shoulder and the top of the dot) to p, the width of the dot's top or printing surface, as described for example in U.S. Pat. No. 8,158,331 to Recchia and U.S. Pat. Pub. No. 2011/0079158 to Recchia et al., the subject matter of each of which is herein incorporated by reference in its entirety. For a truly round-tipped dot, it is difficult to define the exact printing surface because there is not really an edge in the commonly understood sense, and the ratio of re:p can approach 50%. In contrast, a sharp-edged dot would have a very small value of re, and re:p would approach zero. In practice, an re:p of less than 5% is preferred, with an re:p of less than 2% being most preferred.
In the pursuit of printing dots with improved shapes and improved printing performance, photopolymer printing plates have been introduced with photoinitiators with particular chemical and performance characteristics. These new classes of photoinitiators, however, have created printing plates with increased sensitivity to normal ambient daylight and normal indoor fluorescent lighting. These new printing plates have a tendency to cure relatively quickly in the presence of normal ambient daylight and/or indoor fluorescent lighting.